Frequently, underground liquid hydrocarbon reservoirs are associated with gas caps which often serve the beneficial purpose of providing a drive mechanism for displacing the liquid hydrocarbon reserves to the surface through the production string. Gas caps can also form when oil production leads to a drop in reservoir pressure below the bubble point of the oil. Such gas caps are usually comprised of lighter hydrocarbons, primarily in the C1 through C10 carbon number range, and are present in a semi-equilibrium state with the bulk hydrocarbon liquids. Consequently, the actual composition of the gas cap will depend on the temperature and pressure of the reservoir and the composition of the liquid hydrocarbon phase in contact with the the gas cap.
It is not uncommon for the same liquid hydrocarbon reservoir to be in contact with a bottom water zone as well as a gas cap. Consequently, quite often a liquid hydrocarbon reservoir will be sandwiched between a gas cap above and a water zone below with both the gas and water capable of playing beneficial roles. For example, the expansion of the gas cap can be exploited to provide driving force for pushing the liquid hydrocarbons out of the underground reservoir. Similarly, if the water zone is being energized by an underground aquifer, then the energy of the aquifer and the density difference between water and hydrocarbon can be exploited to move liquid hydrocarbons out of the formation.
Understandably, there is a considerable incentive to recover the liquid hydrocarbons in the reservoir as quickly as possible in order to maximize the return on the original investment. Quick recovery usually requires a high drawdown rate. A high drawdown rate creates a high differential pressure between the producing well and the bulk reservoir, which in turn often leads to gas and water coning. Coning is a well condition in which the gas and water layers that bracket the oil zone start to flow toward the wellbore along vertical channels causing undesirable production of water and gas along with the desired hydrocarbon fluids. Once coning begins, increasing quantaties of water and gas are produced causing the ratio of produced oil to produced gas and or water to decrease. Once the cost of separating the produced oil from the produced water and/or gas exceeds the value of the oil, economics dictate shutting down the well.
Although numerous methods have been proposed and tried for restricting flow of gas and water into producing well perforations, none have been particularly successful. Such methods generally are directed to injecting some permeability reducing agent into the formation above and/or below the oil producing zones that would serve to restrict the flow of either gas or water to the producing perforations. Injected permeability reducing agents have always comprised a liquid phase carrying some viscosifying or gelling or cementing agent that serves to transform the injected phase into a thick barrier that limits the flow of gas or water to the oil producing perforations once the fluid has been put in place and the thickening mechanism activated.
Because the permeability reducing or plugging agent is introduced using a liquid phase transport medium, the accurate placement and in-depth propagation of the plug can be difficult. Additionally, once the plug has set, further treatment to abate gas or water coning at some later stage of production is difficult because it invariably requires corrective action beyond the existing plugged zone, while accessing this region is made difficult because of the presence of the original plug. Consequently, there is a need for a more flexible plug delivering mechanism which is capable of deeper penetration into the formation and does not suffer from the limitations of using a liquid phase.
My earlier patent, U.S. Pat. No. 5,095,984 offers a unique mechanism for in-depth delivery of a plugging agent to a high permeability thief zone using a compressed gas phase. This patent, incorporated herein by reference, basically teaches that any combination of compressed gas, cosolvent and polymer or surfactant that has been adjusted to be one phase at some specific temperature and pressure conditions, as defined by some specific application or reservoir properties, can be made to deliver the polymer or surfactant in a form that will plug an oil bearing formation if the temperature of the original mixture is raised or the pressure lowered from the conditions where the system has been made one phase.